Hot water entering a low pressure line (low pressure discharge) is described. Which option represents the cause of water hammer for this scenario?

• A. Cause of Water Hammer
• B. Rapid valve actuation (valve slam)
• C. Filling of a voided line (column rejoining)
• D. Hot water entering a low pressure line (low pressure discharge)

Answer: (A)

Water hammer happens when a sudden change in the flow of liquid creates a pressure surge that travels through the pipe. Here, hot water entering a low‑pressure discharge causes a rapid disturbance in the flowing fluid; the momentum of the incoming water cannot be instantly balanced by an immediate pressure increase everywhere, so a high-pressure transient develops and propagates along the line. That surge is the water hammer itself, so naming the cause as the cause of water hammer points directly to the mechanism: a rapid flow change producing a pressure wave. Specific triggers like valve slam or vapor‑column dynamics are different ways hammer can arise in other cases, but the underlying idea is the abrupt flow change generating the pressure surge.